Dan Whitehorse is Ethan's brother. Laurel comments that Ethan is most likely to be the school 's 'bad boy' and least likely to have a cop as a brother. He detests Sutton due to the pranks she pulled on him prior to the series and told Ethan she's bad news.

Dan looks out for his brother while they're living together. He doesn't like Ethan dating Sutton due to her 'Lying Games'. Even when Emma unknown to him took Sutton's place, he still didnt trust her and thinks Ethan dating a girl like Sutton is not a good idea.

He's owed Alec a great deal and sometimes does things Alec asks him to do.

In The Revengers, Dan wakes a hungover Ethan, he leaves for work kissing Theresa goodbye. Dan is later seen asking why Theresa took Alec's case, after putting him away.

In Cheat, Play, Love, Dan and Theresa discover there is mysterious finger prints on Alec's golf bag proving she was framed. They discover that a woman, Jessica Whitmore, was paid by a man to retrieve the tire iron used to kill Derek, an plant it in Alec's golf bag.

In Advantage Sutton, Dan and Theresa try to see if Alec was framed or if he is playing some kind of game with them. They find out someone made the call to Jessica Whitman in Flagstaff when Alec was at the golf coarse

In Much Ado About Everything, Dan proposes to Theresa and she says yes. At there engagement party, Alec asks Dan to still look into Derek's murder even though the cheif called them off the case.

In The Grave Truth, Dan finds his fiance, Theresa, dead.

In To Lie For, Dan figures out that someone at the pool that night that she was found had something to do with it but has no idea he is correct until a necklace is identified to have belonged to Rebecca. He also learns about the twins after threating Alec he would kill him if he wasn't told the truth right then and there. Dan and Ethan also repair their relationship after Ethan decides to be on his brother's side.